"At the end of every cycle you get a year with three Friday the 13ths the year before the last year in the cycle and you also get one on the tenth year of all the cycles," he said.

Two-thousand nine is the tenth year of the cycle that started in 2000.

Ominous Number

Fear of Friday the 13th is rooted in ancient bad-luck associations with the number 13 and the day Friday, said Donald Dossey, a folklore historian and author of Holiday Folklore, Phobias and Fun.

The two unlucky entities ultimately combined to make one super unlucky day.

Dossey traces the fear of 13 to a Norse myth about 12 gods having a dinner party at Valhalla, their heaven. In walked the uninvited 13th guest, the mischievous Loki. Once there, Loki arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.

"Balder died, and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day," Dossey said.

There is also a biblical reference to the unlucky number 13. Judas, the apostle who betrayed Jesus, was the 13th guest to the Last Supper.

Meanwhile, in ancient Rome, witches reportedly gathered in groups of 12. The 13th was believed to be the devil.

Thomas Fernsler, an associate policy scientist in the Mathematics and Science Education Resource Center at the University of Delaware in Newark, said the number 13 suffers because of its position after 12.

According to Fernsler, numerologists consider 12 a "complete" number. There are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel, and 12 apostles of Jesus.

In exceeding 12 by 1, Fernsler said 13's association with bad luck "has to do with just being a little beyond completeness. The number becomes restless or squirmy."

Feared Date

Some people are so paralyzed with superstition on Friday the 13th that they refuse to fly, buy a house, or act on a hot stock tip.

"It's been estimated that [U.S] $800 or $900 million is lost in business on this day because people will not fly or do business they would normally do," said Dossey, who is also the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina.

Among other services, Dossey's organization counsels clients on how to overcome fear of Friday the 13th, a phobia that he estimates afflicts 17 to 21 million people in the United States.

Symptoms range from mild anxiety to full-blown panic attacks. The latter may cause people to reshuffle schedules or miss an entire day's work.

When it comes to bad luck of any kind, Richard Wiseman—a psychologist at the University of Hertfordshire in Hatfield, England—found that people who consider themselves unfortunate are more likely to believe in superstitions associated with bad luck.

"Their beliefs and behavior are likely to be part of a much bigger worldview," he said. "They will believe that luck is a magical force and that it can ruin their lives."

Many triskaidekaphobes, as those who fear the unlucky integer are known, point to the ill-fated mission to the moon, Apollo 13.

As for Friday, it is well known among Christians as the day Jesus was crucified. Some biblical scholars believe Eve tempted Adam with the forbidden fruit on Friday. Perhaps most significant is a belief that Abel was slain by his brother Cain on Friday the 13th.

Where's the 13th Floor?

This fear of 13 can even be seen in how societies are built. More than 80 percent of high-rise buildings lack a 13th floor. Many airports skip the 13th gate. Hospitals and hotels regularly have no room number 13.

On streets in Florence, Italy, the house between number 12 and 14 is addressed as 12 1/2. In France, socialites known as the quatorziens (fourteeners) once made themselves available as 14th guests to keep a dinner party from an unlucky fate.

DePauw University's Dudley said nobody really knows why Friday the 13th is considered by some people as unlucky.

"You got to have something that is unlucky, and somehow they hit on 13," he said. "But all these explanations are just moonshine."